There is recent interest in forming ceramic precursor gel particles of relatively high purity from gels (either organic bearing or inorganic derived sols). Such gel particles can then be fired to form a particulate, high purity ceramic product. In U.S. Pat. No. 4,789,389 to P. M. Schermerhorn et al., a process for forming silica-gel granules, for example, is described which comprises the condensation of an alkyl orthosilicate, such as, tetraethylorthosilicate, to form a gel followed by drying the gel to produce granules having a mean particle size of less than about one millimeter. The drying is performed at a rapid enough rate to cause the gel to fragment into granules having the desired mean particle size.
U.S. Pat. Nos. 4,098,595 and 4,243,422 to A. Lenz et al. teach drying of a silica gel followed by treatment of the dried gel with water and, if necessary, comminution (e.g., by mechanical grinding of the excessively larger granules that may remain).
While it is known to extrude ceramic precursor gels, it generally has not hitherto been appreciated that such a technique can be used to directly form a particulate product. As used herein the term "particulate" is to be construed as meaning a relatively small, discrete unit of the desired ceramic precursor material which is substantially resistant to further reduction in size. This contrasts these materials from extruded pellets, fibers, or other formed bodies which have a substantial propensity for further size reduction. The following disclosures exist in the art in regard to use of extrusion techniques to form such formed bodies rather than particulate compositions:
1. U.S. Pat. No. 3,920,783 to H. Hara et al. describes the extrusion of a kneaded mass to form constant length pellets (e.g., of alumina, silica, silicates, silicon carbide, titania, zirconia, magnesia, etc.). The pellets are formed from a rod from the extruder nozzle when the rod makes contact with a travelling conveyor belt.
2. U.S. Pat. No. 4,166,147 to R. W. Lange et al. mention extrusion in air of a viscous titania sol to form a green fiber which is then heated and fired.
3. U.S. Pat. No. 4,310,441 to H. Alafandi et al. teaches extrusion of a silica gel to form cylindrical pellets which are then dried and heated.
4. U.S. Pat. No. 4,341,663 to H. Derleth et al. indicates that extrusion of a silica gel, for example, can be used to form a shaped body which is then cut so that the ratio of length to diameter is approximately equal. These cut lengths may then be rounded off (e.g., by a rotating drum, rotating disk, or the like) to form essentially spherical shaped bodies.
5. U.S. Pat. No. 4,838,914 to H. Ohta et al. teaches the extrusion of a silica spinning solution into gel fibers in the production of silica glass fibers.
None of the previously discussed five references appear to illustrate the use of an extrusion procedure to directly form a particulate (or granular) ceramic precursor gel product as contrasted to a formed body (e.g., pellet, fiber, or the like) capable of further substantial size reduction.
More recently, U.S. Pat. Nos. 4,767,429 and (in particular) 4,872,895 to J. W. Fleming et al. disclose the mechanical subdivision of a substantially cohesive gel body by forcing the gel body through a screen having a substantially uniformly spaced, rectilinear grid of strands, e.g., of metallic or polymeric wires. Fleming et al. ,895 indicates that the gel particles resulting from the extrusion step shrink during the drying step which follows due to the result of liquid evaporation and that the degree of shrinkage depends upon the drying procedure used. Example 2 mentions drying in a conventional, glass vacuum dryer. Example 3 mentions drying the gel particles in a microwave oven.
U.S. Pat. No. 4,943,425 to Sophia R. Su et al. mentions mechanically crushing a silica gel through a screen followed by drying.
The Fleming et al. and Su et al. patents, although mentioning the extrusion of silica gel through a screen to form a particulate product, do not discuss the requirements of any downstream processing steps (e.g., material handling up to drying) for the wet gel extrudate.